Investments in Real Estate and Real Estate Under Construction - Summary of acquisitions and build-to-suit transactions (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2017 USD ($) building
Investments in Real Estate and Real Estate Under Construction [Line Items]
Initial Cost Basis	$ 587,583
Land and Land Estate	25,870
Building and Improvements	501,547
Lease in-place Value Intangible	62,956
Below Market Lease Intangible	(2,790)
Office Building [Member] | Lake Jackson, Texas [Member]
Investments in Real Estate and Real Estate Under Construction [Line Items]
Initial Cost Basis	70,401
Land and Land Estate	3,078
Building and Improvements	67,323
Lease in-place Value Intangible	0
Below Market Lease Intangible	$ 0
Number of buildings completed | building	4
Developer partner payout	$ 8,000
Office Building [Member] | Charlotte, North Carolina [Member]
Investments in Real Estate and Real Estate Under Construction [Line Items]
Initial Cost Basis	61,339
Land and Land Estate	3,771
Building and Improvements	47,064
Lease in-place Value Intangible	10,504
Below Market Lease Intangible	0
Industrial Property [Member] | New Century, Kansas [Member]
Investments in Real Estate and Real Estate Under Construction [Line Items]
Initial Cost Basis	12,056
Land and Land Estate	0
Building and Improvements	13,198
Lease in-place Value Intangible	1,648
Below Market Lease Intangible	(2,790)
Industrial Property [Member] | Lebanon, Indiana [Member]
Investments in Real Estate and Real Estate Under Construction [Line Items]
Initial Cost Basis	36,194
Land and Land Estate	2,100
Building and Improvements	29,443
Lease in-place Value Intangible	4,651
Below Market Lease Intangible	0
Industrial Property [Member] | Cleveland, Tennessee [Member]
Investments in Real Estate and Real Estate Under Construction [Line Items]
Initial Cost Basis	34,400
Land and Land Estate	1,871
Building and Improvements	29,743
Lease in-place Value Intangible	2,786
Below Market Lease Intangible	0
Industrial Property [Member] | Grand Prairie, Texas [Member]
Investments in Real Estate and Real Estate Under Construction [Line Items]
Initial Cost Basis	24,317
Land and Land Estate	3,166
Building and Improvements	17,985
Lease in-place Value Intangible	3,166
Below Market Lease Intangible	0
Industrial Property [Member] | San Antonio, Texas [Member]
Investments in Real Estate and Real Estate Under Construction [Line Items]
Initial Cost Basis	45,507
Land and Land Estate	1,311
Building and Improvements	36,644
Lease in-place Value Intangible	7,552
Below Market Lease Intangible	0
Industrial Property [Member] | Opelika, Alabama [Member]
Investments in Real Estate and Real Estate Under Construction [Line Items]
Initial Cost Basis	37,269
Land and Land Estate	134
Building and Improvements	33,183
Lease in-place Value Intangible	3,952
Below Market Lease Intangible	0
Industrial Property [Member] | McDonough, Georgia [Member]
Investments in Real Estate and Real Estate Under Construction [Line Items]
Initial Cost Basis	66,700
Land and Land Estate	5,441
Building and Improvements	52,762
Lease in-place Value Intangible	8,497
Below Market Lease Intangible	0
Industrial Property [Member] | Byhalia, Mississippi [Member]
Investments in Real Estate and Real Estate Under Construction [Line Items]
Initial Cost Basis	36,590
Land and Land Estate	1,751
Building and Improvements	31,236
Lease in-place Value Intangible	3,603
Below Market Lease Intangible	0
Industrial Property [Member] | Jackson, Tennessee [Member]
Investments in Real Estate and Real Estate Under Construction [Line Items]
Initial Cost Basis	57,920
Land and Land Estate	1,454
Building and Improvements	49,026
Lease in-place Value Intangible	7,440
Below Market Lease Intangible	0
Industrial Property [Member] | Smyrna, Tennessee [Member]
Investments in Real Estate and Real Estate Under Construction [Line Items]
Initial Cost Basis	104,890
Land and Land Estate	1,793
Building and Improvements	93,940
Lease in-place Value Intangible	9,157
Below Market Lease Intangible	0
LCIF [Member]
Investments in Real Estate and Real Estate Under Construction [Line Items]
Initial Cost Basis	85,656
Land and Land Estate	6,937
Building and Improvements	65,049
Lease in-place Value Intangible	13,670
LCIF [Member] | Office Building [Member] | Charlotte, North Carolina [Member]
Investments in Real Estate and Real Estate Under Construction [Line Items]
Initial Cost Basis	61,339
Land and Land Estate	3,771
Building and Improvements	47,064
Lease in-place Value Intangible	10,504
LCIF [Member] | Industrial Property [Member] | Grand Prairie, Texas [Member]
Investments in Real Estate and Real Estate Under Construction [Line Items]
Initial Cost Basis	24,317
Land and Land Estate	3,166
Building and Improvements	17,985
Lease in-place Value Intangible	$ 3,166